Hurricane Sandy, a late-season Atlantic storm unlike anything seen in more than two decades, slogged slowly toward the U.S. East Coast on Friday after killing at least 41 people as it cut across the Caribbean.

Forecasters said Sandy, with an expanding wind field already 550 miles wide, had begun merging with a polar air mass over the eastern United States, potentially spawning a hybrid “super storm” that could wreak havoc along the East Coast.

“Its structure is evolving as we speak because it’s interacting with this weather feature at higher levels of the atmosphere,” said Todd Kimberlain, a forecaster at the National Hurricane Center in Miami.

“The models are suggesting that the storm could actually become better organized or intensify a little bit, not due to the normal processes than we would expect for a tropical cyclone but more related to this weather feature,” Kimberlain said.

On its current projected track, government forecasters said Sandy could make landfall early next week anywhere between Virginia, Maryland or Delaware up through New York or southern New England.

In New York City, the global financial hub, officials were considering closing down mass transit before the storm hits.Coming in the final weeks before the presidential election on November 6, the storm could throw last-minute campaign travel plans into chaos.